A hallmark of modern graphical user interfaces is that they allow a large number of graphical objects or items to be displayed on a display screen at the same time. Leading personal computer operating systems, such as Apple Mac OS®, provide user interfaces in which a number of windows can be displayed, overlapped, resized, moved, configured, and reformatted according to the needs of the user or application. Taskbars, menus, virtual buttons and other user interface elements provide mechanisms for accessing and activating windows even when they are hidden behind other windows.
With the sophisticated tools available, users are encouraged not only to create and save a multitude of items in their computers, but to revise or otherwise improve on them over time. For example, a user can work with a certain file and thereafter save its current version on a storage device. The next day, however, the user could have had second thoughts about the revisions, or could have come up with new ideas, and therefore opens the file again.
The revision process is usually straightforward if the user wants to add more material to the file or make changes to what is there. But it is typically more difficult for a user who has changed his/her mind about changes that were previously made and wants the file back as it was once before. Application programs for word processing typically let the user “undo” previous edits of a text, at least up to a predefined number of past revisions. The undo feature also usually is configured so that the previously made revisions must be undone in reverse chronological order; that is, the user must first undo the most recently made edit, then the second-most recent one, and so on. If the user saves and closes the document and thereafter opens it again, it may not be possible to automatically undo any previous edits.
Virtually all computer systems generate event information of some form during operation. For example, “file system events” are used in the Apple Mac OS® X (a corresponding PC version is NT ChangeLog) to provide event notifications for all file system activity (e.g., add file, change file, delete file, etc). Some event information can generate a large data stream, and the capacity to store event notifications can be limited. Additionally, a particular event can require some action by the system and can therefore block the flow of events. To address this situation, event notifications are sometimes dropped to make room for incoming events.